1. Field of the Invention
This invention relates generally to apparatus for controlling vehicle speed, and particularly to such apparatus for maintaining a constant speed with minimum fuel consumption in accordance with a dynamic model of a system relating to a controlled object including an internal combustion engine of the vehicle and power transmission mechanism.
2. Prior art
According to a conventional constant-speed control apparatus for vehicles, such as automobiles, the travelling speed of a vehicle is detected by a vehicle speed detecting means, and then the opening degree of the throttle valve of an internal combustion engine of the vehicle is controlled so that the deviation of the detected vehicle speed from a preset constant speed is made zero. One example of such a conventional apparatus is disclosed in Japanese utility model publication after examination 59-33874 and also in U.S. Pat. No. 4,047,507 corresponding to the Japanese patent.
In such conventional control apparatus, it is desired that the vehicle travelling speed does not deviate from the constant speed, and to suppress possible deviation. Furthermore, it is also desired that the amount of fuel consumed by an internal combustion engine is made as small as possible.
However, in such conventional vehicle speed control apparatus, fuel amount control is effected independently of vehicle speed made through the adjustment of the throttle valve, and therefore, when it is intended to increase the response characteristic, interference occurs between these two control systems. More specifically, when vehicle speed lowers, the throttle valve is opened to increase the vehicle speed, and then intake air quantity is increased to increase fuel amount. As a result, the vehicle speed is increased. Meanwhile, the fuel control system reduces the amount of fuel so as to better fuel consumption. Accordingly, engine output is reduced resulting in lower vehicle speed. In this way, the above-mentioned control steps are repeatedly performed resulting in hunting. As a result, the vehicle speed fluctuates around a given value.
In such conventional vehicle speed control apparatus, the response characteristic of the control system is not satisfactory since the control system is not based on dynamic characteristic, such as inertial mass. Therefore, when a vehicle is driven on a hilly road which goes up and down, the vehicle speed drastically varies.